Minimal Determinants for Binding Activated Gα from the Structure of a Gα i1 −Peptide Dimer † , ‡

G-proteins cycle between an inactive GDP-bound state and active GTP-bound state, serving as molecular switches that coordinate cellular signaling. We recently used phage-display to identify a series of peptides that bind Gα subunits in a nucleotide-dependent manner [Johnston, C. A., Willard, F. S., Jezyk, M. R., Fredericks, Z., Bodor, E. T., Jones, M. B., Blaesius, R., Watts, V. J., Harden, T. K., Sondek, J., Ramer, J. K., and Siderovski, D. P. (2005) Structure 13, 1069–1080]. Here we describe the structural features and functions of KB-1753, a peptide that binds selectively to GDP·AlF4−- and GTPγS-bound states of Gαi subunits. KB-1753 blocks interaction of Gαtransducin with its effector, cGMP phosphodiesterase, and inhibits transducin-mediated activation of cGMP degradation. Additionally, KB-1753 interferes with RGS protein binding and resultant GAP activity. A fluorescent KB-1753 variant was found to act as a sensor for activated Gα in vitro. The crystal structure of KB-1753 bound to Gαi1·GDP·AlF4− reveals binding to a conserved hydrophobic groove between switch II and α3 helices, and, along with supporting biochemical data and previous structural analyses, supports the notion that this is the site of effector interactions for Gαi subunits

Structure of Gαi1 Bound to a GDP-Selective Peptide Provides Insight into Guanine Nucleotide Exchange

Heterotrimeric G-proteins are molecular switches that regulate numerous signaling pathways involved in cellular physiology. This characteristic is achieved by the adoption of two principal states: an inactive, GDP-bound and an active, GTP-bound state. Under basal conditions G-proteins exist in the inactive GDP-bound state, thus nucleotide exchange is crucial to the onset of signaling. Despite our understanding of G-protein signaling pathways, the mechanism of nucleotide exchange remains elusive. We employed phage display technology to identify nucleotide-state-dependent Gα binding peptides. Herein, we report a GDP-selective Gα-binding peptide, KB-752, that enhances spontaneous nucleotide exchange of Gαi subunits. Structural determination of the Gαi1/peptide complex reveals unique changes in the Gα switch regions predicted to enhance nucleotide exchange by creating a GDP dissociation route. Our results cast light onto a potential mechanism by which Gα subunits adopt a conformation suitable for nucleotide exchange

A bifunctional Gαi/Gαs modulatory peptide that attenuates adenylyl cyclase activity

AbstractSignaling via G-protein coupled receptors is initiated by receptor-catalyzed nucleotide exchange on Gα subunits normally bound to GDP and Gβγ. Activated Gα·GTP then regulates effectors such as adenylyl cyclase. Except for Gβγ, no known regulators bind the adenylyl cyclase-stimulatory subunit Gαs in its GDP-bound state. We recently described a peptide, KB-752, that binds and enhances the nucleotide exchange rate of the adenylyl cyclase-inhibitory subunit Gαi. Herein, we report that KB-752 binds Gαs·GDP yet slows its rate of nucleotide exchange. KB-752 inhibits GTPγS-stimulated adenylyl cyclase activity in cell membranes, reflecting its opposing effects on nucleotide exchange by Gαi and Gαs